The present invention relates to a GaN-based semiconductor element, such as a GaN-based blue-violet semiconductor laser element, a GaN-based light emitting diode element capable of emitting light in the ultraviolet to red regions, or a GaN-based high frequency electronic device, and to a GaN-based compound semiconductor epitaxially-grown wafer used for forming such a semiconductor element.
A nitride semiconductor containing nitrogen (N) as a group V element has a relatively large band gap and is thus considered a promising material of a short-wavelength light emitting element. Particularly, a gallium nitride (GaN)-based compound semiconductor represented by the general formula AlxGayInzN (where 0xe2x89xa6x, y, zxe2x89xa61, x+y+z=1) has been actively researched, and a blue light emitting diode (LED) element and a green light emitting diode element have already been put into practical use.
Moreover, there is a strong demand for a blue-violet semiconductor laser element having an oscillation wavelength near 400 nm as a light source for a next-generation high-density optical disk such as an HD-DVD (High Definition Digital Versatile Disk), and semiconductor laser elements using a gallium nitride-based semiconductor material has been actively researched and developed.
FIG. 5 is an enlarged perspective view illustrating a portion of an epitaxially-grown wafer (hereinafter referred to simply as xe2x80x9cepi-waferxe2x80x9d) used in a conventional gallium nitride-based compound semiconductor device.
As illustrated in FIG. 5, the orientation of the principal surface of a substrate 101 made of gallium nitride (GaN) has an off-angle of 0.2xc2x0 in the crystal zone axis  less than 1-100 greater than  direction with respect to the (0001) plane. An element layer 102 made of an epitaxially-grown gallium nitride-based semiconductor is formed on the principal surface of the substrate 101.
The surface morphology of the element layer 102 is influenced by steps (not shown) on the surface of the substrate 101, and has a plurality of steps appearing in the same direction (= less than 1-100 greater than  direction). Thus, a plurality of steps are formed in the direction in which the off-angle is provided to the orientation of the principal surface of the substrate 101. Note that in the present specification, the negative symbol xe2x80x9cxe2x88x92xe2x80x9d attached to an index of an orientation or crystal zone axis is used herein for the sake of convenience to represent the inversion of the index that follows the negative symbol.
In a case where a semiconductor laser element is produced by using such an epi-wafer, it is produced so that the laser resonance direction in a cavity of a waveguide and the step direction (= less than 1-100 greater than  direction) are substantially perpendicular to each other. If the resonance direction of the cavity (stripe direction) is aligned with the step direction of the substrate 101 (off-angle direction), the end surfaces of the cavity opposing each other are shifted from each other in height due to the steps, thereby increasing the internal loss (waveguide loss). This increases the threshold current of the laser element, whereby the reliability is also lowered.
Incidentally, Japanese Laid-Open Patent Publication No. 2000-156348 describes a GaN-based compound semiconductor epi-wafer in which an element layer is formed, wherein the element layer has steps in which the width of a flat portion (terrace portion) between adjacent steps is on the order of 100 to 1000 xcexcm.
In the publication, the resonance direction is set to be parallel to the step direction so as to allow for cleavage along the (1-100) M plane of GaN. Therefore, the width of the terrace portion needs to be at least large enough to be cut out as a chip, i.e., to be larger than the length of one side of the chip.
However, with the GaN-based compound semiconductor epi-wafer of the publication, the off-angle needs to be very small, i.e., about 0.3xc2x0 to 0.5xc2x0, so as to form a terrace portion that is large enough to obtain a laser chip.
Therefore, since the off-angle cannot be larger than 1xc2x0, for example, the surface morphology of the epi-wafer may deteriorate, and the half-width of the X-ray rocking curve, which is an index of the orientation of a crystal, may increase, thereby failing to obtain an element layer having a high crystallinity. Therefore, a semiconductor device produced by using the conventional GaN-based compound semiconductor epi-wafer will have insufficient characteristics.
The present invention, which has been made to solve the problems in the prior art, has an object to obtain a high crystallinity in an epitaxial layer (element layer) of a GaN-based compound semiconductor epi-wafer having an off-angle.
In order to achieve the object, a GaN-based compound semiconductor epi-wafer of the present invention includes: a substrate made of a first nitride semiconductor belonging to a hexagonal system; and an element layer for forming a semiconductor element, which is made of a second nitride semiconductor belonging to the hexagonal system and which is grown on a principal surface of the substrate, wherein: an orientation of the principal surface of the substrate has an off-angle in a predetermined direction with respect to a (0001) plane; and the element layer has a surface morphology of a stripe pattern extending substantially in parallel to the predetermined direction.
With the GaN-based compound semiconductor epi-wafer of the present invention, the element layer has a surface morphology of a stripe pattern extending substantially in parallel to the predetermined direction, in which the off-angle is provided, whereby the width between stripes (the width of the terrace portion) will never be narrowed even if the off-angle of the substrate is set to be as large as 1xc2x0 or more. Therefore, the element layer has a desirable surface morphology, and an element layer having a good crystallinity can be obtained. Note that a surface morphology of a stripe pattern is a step that is formed by one or two atomic layers and is present on the surface of the element layer.
In the GaN-based compound semiconductor epi-wafer of the present invention, it is preferred that the predetermined direction is a crystal zone axis  less than 1-100 greater than  direction.
In the GaN-based compound semiconductor epi-wafer of the present invention, it is preferred that the off-angle is equal to or greater than 1xc2x0 and less than or equal to 10xc2x0.
A semiconductor element of the present invention is a semiconductor element using a GaN-based compound semiconductor epi-wafer, the GaN-based compound semiconductor epi-wafer including: a substrate made of a first nitride semiconductor belonging to a hexagonal system; and an element layer made of a second nitride semiconductor belonging to the hexagonal system for forming the semiconductor element, the element layer being grown on a principal surface of the substrate, wherein: an orientation of the principal surface of the substrate has an off-angle in a predetermined direction with respect to a (0001) plane; and the element layer has a surface morphology of a stripe pattern extending substantially in parallel to the predetermined direction.
With the semiconductor element of the present invention, the element layer has a surface morphology of a stripe pattern extending substantially in parallel to the predetermined direction, in which the off-angle is provided, whereby the width between stripes will not be narrowed even if the off-angle of the substrate is set to be as large as 1xc2x0 or more. Therefore, the element layer has a desirable surface morphology, and an element layer having a good crystallinity can be obtained.
In the semiconductor element of the present invention, it is preferred that the predetermined direction is a crystal zone axis  less than 1-100 greater than  direction.
In the semiconductor element of the present invention, it is preferred that the off-angle is equal to or greater than 1xc2x0 and less than or equal to 10xc2x0.
It is preferred that the semiconductor element of the present invention is a laser element.
In such a case, it is preferred that the laser element includes a cavity formed to be substantially parallel to the predetermined direction.
In such a case, it is preferred that: the off-angle is equal to or greater than 1xc2x0 and less than or equal to 10xc2x0; and end surfaces of the cavity are formed by cleavage.
Moreover, it is preferred that the semiconductor element of the present invention is a light emitting diode element.